The present invention pertains generally to the technical field of digital signal processing. More particularly, the present invention pertains to an apparatus and method for asymmetry control in an optical PRML (Partial Response Maximum Likelihood) read channel.
DVD, an acronym for digital video disc or digital versatile disc, is a relatively new type of compact-disc read-only-memory (CD-ROM) with a minimum capacity of approximately 4.7 gigabytes. A typical DVD player includes an optical pick-up unit (OPU), a read channel, and a digital video decoder. The OPU converts information read from the DVD into an analog RF signal. The read channel takes this RF signal and generates a digital data signal and a synchronous clock signal. The read channel couples these signals to the digital video decoder, which decodes the data and converts it into a video format compatible with a TV.
Due to imperfections in the manufacturing and recording processes, DVDs typically contain pits and/or marks with slightly incorrect lengths. The incorrect lengths of pits and/or marks, also known as domain bloom, may contribute to asymmetry in the RF signal. The presence of DC blocking capacitors introduces low frequence disturbances into the RF signal, and further exacerbates the asymmetry problem. If uncompensated, asymmetrical elements of the RF signal can be a major impairment to reliable data detection. Therefore, it is important for a DVD player to have the capability to accurately detect and compensate for such asymmetry.
In some conventional read channels, signal asymmetry is detected by using three digital samples along a signal edge (either positive edge or negative). The amplitude of the center sample is compared against a set threshold to see whether it represents a zero-crossing. If so, the two neighboring samples are then summed to determine the amplitude and polarity of the asymmetry. One problem, however, with this conventional approach is that a sample may be inaccurately identified as a zero-crossing if the amount of low frequency disturbance is significant. As a result, the performance of the read channel is adversely impacted.
Therefore, what is needed is an improved asymmetry control apparatus and method that is not sensitive to the amount of low frequency disturbance in the signal. What is also needed is an apparatus and method for detecting zero-crossings of a signal without comparing the center sample against a set threshold.
Accordingly, the present invention provides an improved method and apparatus for detecting zero-crossings of a signal. Significantly, according to the present invention, a zero-crossing of a signal is detected by calculating the differences between neighboring samples, and by comparing the differences against pre-determined threshold value(s). Since the present invention does not directly compare signal samples against pre-determined thresholds, the present invention is independent of low frequency disturbances, baseline wandering and DC offset of the signal.
In one embodiment, the method of the present invention includes the step of receiving digital samples of an RF signal that has a first data value (d0), a second data value (d1) and a third data value (d2), and the step of calculating the differences between neighboring samples. Particularly, a first difference (d1xe2x88x92d0) is calculated by subtracting the first data value from the second data value; and, a second difference (d2xe2x88x92d0) is calculated by subtracting the second data value from the third data value. The first and second differences are then compared against a pre-determined threshold value. If both differences exceed the pre-determined threshold value, then it can be concluded that the center sample (d1) is a zero-crossing sample. An asymmetry offset value based on the first data value and the third data value can then be determined.
In another embodiment of the present invention, zero-crossing samples may be identified by comparing the differences to a pre-determined negative threshold value when the samples represent a negative edge. In that case, if both the first and second differences are smaller than the pre-determined negative threshold value, then it can also be concluded that the center sample is a zero-crossing sample.
The present invention also provides a zero-crossing circuit which may be implemented as part of an optical PRML read channel. In this embodiment, the optical PRML read channel includes an analog-to-digital converter for converting an RF signal into a digital signal, and an asymmetry control circuit for correcting asymmetry of the digital signal. The asymmetry control circuit of the present embodiment includes circuits for receiving samples of the digital signal, circuits for identifying a zero-crossing sample by calculating differences between neighboring samples and comparing the differences to pre-determined threshold value(s), and circuits for generating an asymmetry offset voltage based on the values of the digital samples, provided a zero-crossing sample is detected.